Afterslip following the 2007 M_w 8.4 Bengkulu earthquake in Sumatra loaded the 2010 M_w 7.8 Mentawai tsunami earthquake rupture zone

Abstract

The 12 September 2007 Mw 8.4 Bengkulu earthquake in Sumatra marked the first in a modern series of large earthquakes along the Mentawai section of the Sunda megathrust. Understanding the spatial distribution of coseismic slip and ensuing afterslip is important for assessing seismic hazard in neighboring unruptured regions of the megathrust. We reestimate the spatial distribution of coseismic slip during this earthquake with improved coseismic offsets from the Sumatran GPS Array (SuGAr) and estimate afterslip following this earthquake with SuGAr postseismic time series spanning ∼6.3 years after the earthquake. We invert for the spatiotemporal distribution of afterslip with the principal component analysis‐based inversion method (PCAIM), and we take into account viscoelastic deformation by incorporating into the inversion the estimation of strain within ductile deforming blocks located at asthenospheric depths. Our results suggest cumulative afterslip concentrated within, updip, and downdip of the 2007 coseismic rupture area and shallow afterslip that borders and overlaps the 2010 Mw 7.8 Mentawai earthquake rupture zone. The cumulative contribution of stress changes due to the coseismic event and the ensuing afterslip likely increased strain rates in the shallow portion of the megathrust adjacent to the Mentawai earthquake rupture area, potentially promoting its rupture in 2010.

Piecemeal Rupture of the Mentawai Patch, Sumatra: The 2008 Mw 7.2 North Pagai Earthquake Sequence

“…We show that the estimated afterslip following the 2008 event partially overlaps the updip region of its coseismic rupture area. Similar overlapping cases were also inferred for afterslip following the 2011 Tohoku-Oki earthquake (Johnson et al, , 2016, the 2010 Maule earthquake (Bedford et al, 2013), the 2015 Illapel earthquake (Barnhart et al, 2016;Shrivastava et al, 2016), the 2007 Bengkulu earthquake (Tsang et al, 2016), and the 2010 Mentawai earthquake . In the Mentawai patch, afterslip following the recent earthquakes does seem to have a preference to overlap.…”

“…To the contrary, the northern half of the Mentawai patch has experienced only M ∼7 earthquakes including aftershocks of the 2007 Bengkulu earthquake (M w 7.9 and 7.0) (Konca et al, 2008;Tsang et al, 2016) and the 2008 M w 7.2 North Pagai earthquake.…”

“…The southern half of the Mentawai patch ruptured during the 2007 M w 8.4 Bengkulu earthquake (Konca et al, 2008;Tsang et al, 2016) and the 2010 M w 7.8 Mentawai tsunami earthquake Lay et al, 2011;Yue et al, 2014) (Figure 1). These events and their following afterslip Lubis et al, 2012) have released only ∼30-44% of the moment (Tsang et al, 2016) that was released during the 1833 event.…”

“…This sequence of earthquakes ruptured a small area in the middle of the Mentawai patch ( Figure 1) and has not been extensively studied to date. The fact that only a small area of the Mentawai patch has ruptured so far could have important implications for understanding the unbroken megathrust in the northern half of the Mentawai patch, particularly (Tsang et al, 2016) and the 2010 M w 7.8 Mentawai tsunami earthquake (green area) . The red star indicates the epicenter of the 2008 mainshock, which was preceded by four M ∼5 foreshocks 1 week before and one M w 6.5 foreshock the day before (gray focal mechanisms).…”

Asthenosphere Flow Modulated by Megathrust Earthquake Cycles

“…This is accomplished using the integral method, combining boundary and volume elements to resolve all phases of the seismic cycle under rate-and-state friction-with the notable exception of the radiation of seismic waves-and the mechanical coupling between brittle and ductile deformation. The displacement and stress kernels for a volume element can be used to directly image the anelastic deformation in Earth's interior Qiu et al, 2018;Tsang et al, 2016) and to simulate forward models of deformation (Goswami & Barbot, 2018;Lambert & Barbot, 2016). It was extended to incorporating shear heating and heat diffusion by Goswami and Barbot (2018).…”